Hi, John D,
Stans is correct, but nearly all Colts with the old mechanism will fail that test. Do as suggested, then pull the trigger. The cylinder should lock up. Take a look at the fired cases. If the firing pin mark is centered, the gun can't be too far out of time.
The advice on replacing the hand is good, but there are a couple of other possibilities to look at. One is the forcing cone in the barrel to see if it is tapered at the proper angle. Another is to use a range rod to be sure the chambers are lining up when the cylinder is locked. The barrel-cylinder gap can also play a part in lead spitting, as can the bullets themselves (hardness, size).
Frankly, if the hand needs to be replaced, I would have to recommend taking the gun to a gunsmith. Working on the old Colts is tricky and not really a place for the DIYer to start.
Jim